At present, as the majority of silicon single crystal ingots used for manufacturing a substrate for forming a circuit component of a LSI (large scale integrated circuit) and the like, silicon single crystals pulled by the CZ method have been used. FIG. 8 is a diagrammatic sectional view of a conventional apparatus for pulling a single crystal used for the CZ method, and in the figure, reference numeral 61 represents a crucible.
The crucible 61 comprises a bottomed cylindrical quartz crucible 61a and a bottomed cylindrical graphite crucible 61b fitted on the outer side of the quartz crucible 61a and supporting the quartz crucible 61a. The crucible 61 is supported with a support shaft 68 which rotates in the direction shown by the arrow in the figure at a prescribed speed. A cylindrical heater 62 of a resistance heating type and a cylindrical heat insulating mould 67 are concentrically arranged around the crucible 61. The crucible 61 is charged with a melt 63 of a material for forming a crystal which is melted by the heater 62. On the central axis of the crucible 61, a pulling axis 64 made of a pulling rod or wire is suspended, and at the lower end portion thereof, a seed crystal 65 is held by a holder 64a. Each part mentioned above is arranged at a fixed place in a water cooled type chamber 69 wherein pressure and temperature can be controlled.
A method for pulling a single crystal 66 using the above-mentioned apparatus for pulling a single crystal is described below by reference to FIGS. 8 and 9. FIGS. 9(a)-(d) are partial magnified front views diagrammatically showing the seed crystal and the vicinity thereof in part of the steps in pulling a single crystal.
Although it is not shown in FIG. 9, the material for forming a crystal is melted by the heater 62. The pressure in the chamber 69 is reduced and is maintained for a period of time so as to sufficiently release gas contained in the melt 63. Then, an inert gas is induced into the chamber 69 from the upper part thereof so as to make an inert gas atmosphere under reduced pressure within the chamber 69.
While the pulling axis 64 is rotated on the same axis in the reverse direction of the support shaft 68 at a prescribed speed, the seed crystal 65 held by the holder 64a is caused to descend. The front portion 65a thereof is brought into contact with the surface of the melt 63 so as to make the seed crystal 65 partially melt into the melt 63 (hereinafter, referred to as the seeding step) (FIG. 9(a)).
In making a crystal grow at the front of the seed crystal 65, the pulling axis 64 is pulled at a higher speed than the below-described pulling speed in forming a main body 66c. The crystal is narrowed down to have a prescribed diameter, leading to the formation of a neck 66a (hereinafter, referred to as the necking step) (FIG. 9(b)).
By slowing down the pulling speed of the pulling axis 64 (hereinafter, simply referred to as the pulling speed), the neck 66a is made to grow to have a prescribed diameter, leading to the formation of a shoulder 66b (hereinafter, referred to as the shoulder formation step) (FIG. 9(c)).
By pulling the pulling axis 64 at a fixed rate, the main body 66c having a uniform diameter and a prescribed length is formed (hereinafter, referred to as the main body formation step) (FIG. 9(d)).
Although it is not shown in FIG. 9, in order to prevent induction of high density dislocation to the single crystal 66 by a steep temperature gradient, the diameter thereof is gradually decreased and the temperature of the whole single crystal 66 is gradually lowered, leading to the formation of an end-cone and a tail end. Then, the single crystal 66 is separated from the melt 63. Cooling the single crystal 66 is at the end of the pulling of the single crystal 66.
One of the most important steps in the pulling of the single crystal 66 is the above-mentioned necking step (FIG. 9(b)). In the seeding step (FIG. 9(a)), the front portion 65a of the seed crystal 65 is preheated to some extent and is brought into contact with the melt 63. There is usually a difference of 100.degree. C. or more between the preheating temperature (about 1300.degree. C. and less) and the melting point of the seed crystal 65 (about 1410.degree. C.). Therefore, in dipping the seed crystal 65 into the melt 63, the seed crystal 65 has a steep temperature gradient, leading to the induction of the dislocation caused by a thermal stress to the front portion 65a of the seed crystal 65. It is necessary to make the single crystal 66 grow after excluding the dislocation which inhibits single crystal growth. Since the dislocation generally tends to grow in the vertical direction to the growth interface of the single crystal 66, the shape of the growth interface (the front plane of the neck 66a) is made to be downward convex in the necking step, so as to exclude the dislocation.
In the necking step, the faster the pulling speed is made, the smaller the diameter of the neck 66a becomes and the more downward convex the shape of the growth interface becomes. As a result, the dislocation is inhibited from propagating and can be efficiently excluded.
In the above conventional method for pulling a single crystal, the seed crystal 65 having a diameter of about 12 mm has been generally used in order to pull the single crystal 66 having a diameter of about 6 inches and a weight of 80 kg or so. The larger diameter of the neck 66a is, the more safely the single crystal 66 is held, while the smaller diameter of the neck 66a is, the more efficiently the dislocation is excluded. In order to meet both of the requirements, the neck 66a having a diameter of about 3 mm is selected.
Recently, however, in order to produce a single crystal ingot at a lower cost and more efficiently, and to improve the yield of chips, the wafer has been required to have a larger diameter. Now, for example, the production of the single crystal 66 having a diameter of about 12 inches (300 mm) and a weight of 300 kg or so is desired. In this case, the neck 66a having a conventional diameter (usually 3 mm or so) cannot withstand the weight of the pulled single crystal 66 and breaks, resulting in the falling of the single crystal 66.
In growing the above heavy single crystal 66, the diameter of the neck 66a needs to be about 6 mm or more in order to prevent the occurrence of troubles such as a fall of the single crystal 66 and to pull the single crystal 66 safely, which is calculated from the silicon strength (about 16 kgf/mm.sup.2). However, when the diameter of the neck 66a is 6 mm or more, the dislocation induced in dipping the seed crystal 65 into the melt 63 cannot be sufficiently excluded, leading to the induction of the dislocation to the pulled single crystal 66.